Very good point. Mexico is not noted for its hygiene.
Best advice I can say is stay away from those traveling to Mexico and ILLEGALS that go back and forth. Last I hear, this Mexican Swine Flu originated in Mexico.
That's also a consideration: your immune system is better able to fight off one infection at a time. When it is continually having to deal with stuff that comes in from an unsanitary environment, it's unlikely to have enough spare capacity to deal with an aggressive viral infection.